public class Solution {
    public bool IsPalindrome(string s) {
        int i = 0;
        int j = s.Length - 1;

        while (i < j)
        {
            if (!Char.IsLetterOrDigit(s[i]))
            {
                i++;
            }
            if (!Char.IsLetterOrDigit(s[j]))
            {
                j--;
            }
            if (Char.IsLetterOrDigit(s[i]) && Char.IsLetterOrDigit(s[j]))
            {
                if (Char.ToLower(s[i]) != Char.ToLower(s[j]))
                {
                    return false;
                }
                else
                {
                    i++;
                    j--;
                }
            }
        }

        return true;
    }
}

class Solution {
    // time O(n)
    // space O(1)
    public boolean isPalindrome(String s) {
        int i = 0;
        int j = s.length() - 1;

        while (i < j) {
            // The String.toLowerCase() method in Java has a time complexity of O(n), where n is the length of the string. Given n is always 1, O(1) here.
            char ci = Character.toLowerCase(s.charAt(i));
            char cj = Character.toLowerCase(s.charAt(j));

            if (!Character.isLetterOrDigit(ci)) {
                i++;
            }
            else if (!Character.isLetterOrDigit(cj)) {
                j--;
            }
            else if (ci != cj) {
                return false;
            }
            else {
                i++;
                j--;
            }
        }
        return true;
    }
}